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Heat Treating steel
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The object in heating is to transform pearlite (a mechanical mixture of iron carbide that exists in a finely mixed condition) to austenite as the steel transition takes time, a relatively slow rate of heating must be used. Ordinarily, the cold steel is inserted when the temperature in the furnace is from 300*F, to 500*F. below the hardening temperature. In this way, too rapid heating through the critical range is prevented. If temperature-measuring equipment is not available, it becomes necessary to estimate temperatures by some other means. An inexpensive, yet fairly accurate method involves the use of commercial crayons, pellets, or paints that melt at various temperatures within the range of 125*F. to 1600*F. The least accurate method of temperature estimation is by observation of the color of the hot hearth of the furnace or of the work. The heat colors observed are affected by many factors, such as the conditions of artificial or natural light, the character of the scale on the work, etc.
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Steel begins to appear dull red at about 1000*F, and as the temperature increases the color changes gradually through various shades of red to change, to yellow, and finally to white. A rough approximation of the correspondence between color and temperature. It also possible to secure some idea of the temperature of a piece of carbon or low-alloy steel, in the low temperature range used for tempering, from the color of the thin oxide film that forms on the cleaned surface of the steel when heated in this range. The approximate temperature-color relationship for a time at temperature of about one-half is indicated.
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